The present invention generally relates to a semiconductor device, and more particularly relates to a semiconductor device including a fluorine-containing insulating film, made of an insulator doped with fluorine, between metal interconnects and a method for fabricating such a device.
Hereinafter, a semiconductor device including a fluorine-containing insulating film, made of an insulator doped with fluorine, between metal interconnects will be described with reference to FIG. 9.
As shown in FIG. 9, an insulating film 2, made of silicon dioxide, is formed on a semiconductor substrate 1. On the insulating film 2, a plurality of lower-level metal interconnects 3 are formed. Each of these metal interconnects 3 has a multilayer structure formed by stacking a first titanium film 3a, a first aluminum alloy film 3b and a first titanium nitride film 3c in this order. A fluorine-containing silicon dioxide film 4 is formed by doping a silicon dioxide film with fluorine to fill in the gaps between adjacent ones of the lower-level metal interconnects 3 and to cover all of these metal interconnects 3.
An ordinary silicon dioxide film 5 is formed on the fluorine-containing silicon dioxide film 4. And on the silicon dioxide film 5, a plurality of upper-level metal interconnects 6 are formed. Each of these metal interconnects 6 also has a multilayer structure formed by stacking a second titanium film 6a, a second aluminum alloy film 6b and a second titanium nitride film 6c in this order.
The relative dielectric constant of the fluorine-containing silicon dioxide film 4, formed by doping a silicon dioxide film with fluorine, is lower than that of the ordinary silicon dioxide film 5, which is not doped with fluorine. Thus, by providing an interlevel insulating film including the fluorine-containing silicon dioxide film 4 to fill in the gaps between the lower-level metal interconnects 3 and cover all of these interconnects 3, parasitic capacitances between the interconnects 3 themselves and between the lower- and upper-level metal interconnects 3, 6 can be reduced. As a result, signals can be transmitted with a reduced delay, thus enabling the use of signals with a higher frequency.
However, if the fluorine-containing silicon dioxide film 4 is interposed between the lower- and upper-level metal interconnects 3, 6, then fluorine, contained in the fluorine-containing silicon dioxide film 4, is likely to diffuse into the silicon dioxide film 5 during a subsequent heat treatment process. Such a phenomenon is called "auto-doping" in the pertinent art. Once fluorine has diffused into the silicon dioxide film 5, a degraded layer is formed in the interface between the second titanium film 6a, which is the lowermost layer of the upper-level metal interconnects 6, and the silicon dioxide film 5. Accordingly, the upper-level metal interconnects 6 come to make poorer contact with the silicon dioxide film 5, resulting in deterioration in reliability of the semiconductor device.